1. Field of the Invention
This invention relates generally to microelectromechanical system (MEMS) microsensors, and in particular to a sensor and method for making a sensor that operates using a relative movement between a magnetoresistive element, a movable microstructure such as a bridge, diaphragm, membrane, or cantilever beam, and a hard magnetic film. The device functions to interpret a relative displacement of the movable microstructure with respect to the MR or GMR element from a detected change in the local magnetic field, which in turn can be used to measure acceleration, pressure, temperature, or vibration.
2. Description of Related Art
Microelectromechanical sensors (MEMS) have been previously based on various physical principles such as piezoelectricity, tunneling, elasticity, capacitance, optical interference, and so forth. For example, a measurement of acceleration is required for many everyday applications such as guidance control, detonation, shock, and vibration measurement. The technology for measuring acceleration has included electromechanical, piezoelectric, piezoresistive, and capacitive acceleration sensors. There are physical limitations of each of the aforementioned technology which limit the sensitivity and, consequently, the accuracy of these accelerometers.
All accelerometers are fundamentally displacement measuring devices that operate using the conversion of acceleration into a force, causing a displacement, which is then turned into an electrical signal. This displacement is resisted by a calibrated spring or its functional equivalent. Using Newton""s second law of physics and Hooke""s law of springs, an equation can be derived relating acceleration to displacement:
F=maxe2x80x83xe2x80x83{1}
F=xe2x88x92kxxe2x80x83xe2x80x83{2}
∴a=xe2x88x92kx/mxe2x80x83xe2x80x83{3}
Where F=force, m=Mass, a=Acceleration, k=spring constant, and x=displacement. Equations describing accelerometers are simple and well known. Accelerometers measure xe2x80x9caxe2x80x9d by knowing xe2x80x9ckxe2x80x9d and xe2x80x9cmxe2x80x9d, observing xe2x80x9cxxe2x80x9d and using Eq {3}.
Some current microelectromechanical sensors operate under tunneling tip technology. Aside from the uncertainty issues concerning the electrical behavior of tunneling tip technology, as well as manufacturing problems, these designs require tip spacing of approximately 1 nanometer. Establishing this spacing is difficult, unreliable once set, and problematic to maintain during operation. Problems associated with impact during tip setting leads to rejection of parts as a result of the damage caused.
The Magnetoresistive Effect
The xe2x80x9cMagnetoresistive (MR) effect was discovered in perfect-crystal samples exposed to very high magnetic fields. The effect was also recently discovered in sputtered metallic thin films consisting of magnetic layers a few nanometers thick separated by equally thin nonmagnetic layers (Giant Magnetoresistive elements, or xe2x80x9cGMRxe2x80x9d). A large decrease in the resistance of these films is observed when a magnetic field is applied. The cause of this effect is the spin dependence of electron scattering and the spin polarization of conduction electrons in ferromagnetic metals. With layers of the proper thickness, adjacent magnetic layers couple antiferromagnetically to each other with the magnetic moments of each magnetic layer aligned antiparallel to the adjacent magnetic layers.
A change in the magnetic orientation of the sensing layer will cause a change in the resistance of the combined sensing and pinned layers. The GMR sensor materials have two spin states, known as xe2x80x9cspin upxe2x80x9d and xe2x80x9cspin down.xe2x80x9d Conduction electrons with a spin direction parallel to a material""s magnetic orientation move freely, producing a lower electrical resistance. Conversely, conduction electrons with a spin direction opposite to the materials"" magnetic orientation are hampered by more frequent collisions with atoms in the material, resulting in a higher electrical resistance. The size of this decrease in resistivity can be 10% to 20% and higher in GMR materials with multiple nonmagnetic layers.
The present invention is an ultrasensitive displacement determining sensor which employs a sputter deposited, multilayer magnetoresistive or giant magnetoresistive field sensor. For the purposes of this disclosure, it is to be understood that the invention may be practiced with either an MR or a GMR element, and that the term xe2x80x9cmagnetoresistive elementxe2x80x9d refers generally to either type. For illustration purposes, the GMR element will be described below. The sensor includes a micromachined microstructure, such as a membrane, cantilever beam, or bridge, diaphragm, with a sputtered, hard magnetic film deposited on the microstructure. The GMR sensor detects displacement by sensing changes in magnetic field caused by the movement of the hard magnetic film on the microstructure.
In one preferred embodiment, utilizing a bulk micromachining approach, very thin (0.5-1 xcexcm) silicon and silicon nitride membranes are fabricated by means of anisotropic etching of silicon wafers and/or reactive ion etching of silicon on insulator (SOI) or Low Pressure Chemical Vapor Deposited (LPCVD) silicon nitride films over silicon substrate. A hard magnetic thin film is deposited over the silicon nitride microstructure to impose a magnetic field on the GMR element. The GMR element is a multilayer device comprising a sensing layer, conducting spacer, a pinned layer, and an exchange layer, and space layers to avoid the diffusion between the sensing layer and conducting spacer. By passing a current through the GMR sensor and monitoring the electrical characteristics of the sensor, the magnetic field, and thus the displacement of the microstructure, can be determined.
The thickness of each layer of the GMR sensor is very thinxe2x80x94on the order of angstroms (xc3x85)xe2x80x94except for the exchange layer which will allow the conduction of electrons to frequently move back and forth between the sensing and pinned layers via the conducting spacer. The magnetic orientation of the pinned layer is fixed and held in place by the adjacent exchange layer, while the magnetic orientation of the sensing layer changes in response to the external magnetic field.
The GMR sensor directly detects the magnetic field, and is sensitive to small changes in the magnetic field. The sensor is especially suited to measure position or displacement in linear and rotational systems. One application involves the placement of a sensor on a tunneling tip to accurately place the tip with respect to the substrate. Because such devices are manufactured using established thin film deposition of magnetic materials used in the data storage industry, many processing problems have already been addressed. Moreover, because the field may be varied depending on film thickness, potential for damage during fabrication can be alleviated by maintaining relatively significantly greater spacing than is currently used. Furthermore, the detection of magnetic field strength is a very well understood and monitored phenomenon, nanometer and sub-nanometer displacements can accurately be measured.